High demands are placed on glasses for high-temperature lamps, which are generally understood to mean lamps having bulb temperatures of above 550.degree. C.
The glasses in question are alumino(boro)silicate glasses which contain alkaline earth metals. The glasses must be essentially free of alkali metal oxides, since alkali metal ions interfere with the regenerative halogen cycle in the lamp. This is because, during operation of the lamp, the tungsten vapor from the filament and the halogen/inert gas mixture form an equilibrium between formation and decomposition of tungsten halides, the decomposition reaction taking place at higher temperatures than the formation reaction, so that the tungsten is deposited back on the filament. If this cycle is interfered with by contaminating components, such as for example alkali metal ions, the tungsten is deposited not on the filament but on the inside of the glass bulb, as an undesirable shiny black disruptive coating.
The patent literature already contains numerous documents which relate to glasses for incandescent lamps. However, these glasses are afflicted with a very wide range of drawbacks.
U.S. Pat. No. 3,978,362 describes incandescent lamps with lamp bulbs made from glass which has a high CaO content (14-21% by weight).
German patent publication 3,736,887 describes incandescent lamps made from glass bulbs which have a high BaO content (10-23% by weight).
Other documents claim glasses which have specific ratios between CaO and BaO.
German patent publication 2,733,169 relates to glasses for seals with molybdenum in which the CaO : BaO weight ratio is between 0.6 and 1. These glasses are B.sub.2 O.sub.3 -free.
German patent publication 2,930,249 relates to the use of specific glass compositions as bulb material, in which compositions the BaO:CaO weight ratio is between 2.3 and 3.5 (CaO:BaO between 0.28 and 0.43). The glasses described in the latter document are said to have an improved resistance to what is known as "reboil". Reboil is the tendency of the glass to form a large number of small bubbles of included gases when subjected to further processing with a flame or when reheated. This impairs the light transmission and weakens the reheated areas.
A drawback of the glasses which have very high BaO contents are the high melting and working temperatures, which lead to increased stress on the tank furnace material and entail high power costs, while the glasses which have high CaO contents have a greater tendency towards crystallization and a higher expansion coefficient compared to those which have a high BaO content.
U.S. Pat. No. 4,060,423 describes B.sub.2 O.sub.3 -free glasses for Mo seals in which the Al.sub.2 O.sub.3 /(BaO+CaO) weight ratio lies in the range from 0.6 to 1.
U.S. Pat. No. 4,298,388 also describes B.sub.2 O.sub.3 -free glasses for glass-Mo seals. These glasses are also MgO-free and have high CaO contents (up to 19.2% by weight). BaO is only an optional component.
Although the B.sub.2 O.sub.3 -free glasses, at least if they contain little or no MgO, have sufficient thermal stability, as is evidenced, for example, by high softening temperatures and high strain points, and in principal also allow bulb temperatures of up to 700.degree. C., they are subject to the drawback that they have to be melted at high temperatures. The associated high level of corrosion to the tank blocks and the crown material to some extent leads to poor quality of the glass and to an increase in the impurity level, which means that such a glass is not particularly suitable for use as a bulb material, since increasing levels of impurities may lead to a higher level of black discoloration in the lamp.
Glasses which require large quantities of B.sub.2 O.sub.3 for incandescent lamp bulbs are also known. For example, the glasses for seals with molybdenum described in U.S. Pat. No. 3,310,413 contain 4 to 9% by weight of B.sub.2 O.sub.3. The sealing or bulb glasses described in DE 33 05 587 A1 also require 3 to 7% by weight of B.sub.2 O.sub.3 and, moreover, high BaO contents (11-16% by weight). Such high B.sub.2 O.sub.3 contents, in particular in combination with MgO, reduce the viscosity values, so that these glasses are unsuitable for halogen lamps with bulb temperatures of greater than 650.degree. C., for example of approx. 700.degree. C. The low stability of the glasses at high temperatures leads to the lamp bulb bulging, possibly even to such an extent that the bulb will explode. One example of such a glass is the commercially available glass V1, having the composition (in % by weight) 56.8 SiO.sub.2 ; 16.4 Al.sub.2 O.sub.3 ; 4.7 B.sub.2 O.sub.3 ; 5.8 MgO; 7.8 CaO; 8.0 BaO, with an annealing point AP of 721.degree. C.
U.S. Pat. No. 3,496,401 describes incandescent lamps made from an aluminosilicate glass containing alkaline earth metals and having a maximum alkali metal oxide content of 0.1% by weight, in particular made from glasses comprising SiO.sub.2, Al.sub.2 O.sub.3, 10-25% by weight of alkaline-earth metal oxides, the specific levels of which are not given in further detail, and 0-10% by weight of B.sub.2 O.sub.3. The exemplary embodiments are either B.sub.2 O.sub.3 -free or contain at least 4% by weight of B.sub.2 O.sub.3. The maximum permissible alkali metal oxide content is too high for the high bulb temperatures of approx. 700.degree. C. and, during operation of the lamp, will lead to blackening of the internal surface of the bulb.
The bulb temperature increases at high lamp powers. Since, as the temperature increases, the ion mobility in the glass increases and diffusion processes proceed more easily, even relatively small quantities of alkali metal ions are sufficient to interfere with the halogen cycle.
Like the alkali metal ions, water or hydrogen ions also have an adverse effect on the halogen cycle.
For this reason, U.S. Pat. No. 4,163,171 describes an incandescent lamp in which the glass is not only "essentially free from alkali metals", but may also only contain less than 0.03% by weight of water.
U.S. Pat. No. 5,489,558 describes glasses which are suitable in particular for use in flat panel displays. These glasses either have high Al.sub.2 O.sub.3 contents (.gtoreq.18% by weight) combined with relatively low SiO.sub.2 contents (.ltoreq.55% by weight) or low Al.sub.2 O.sub.3 contents (.ltoreq.13% by weight) combined with higher SiO.sub.2 contents (.gtoreq.55% by weight). The glasses are specifically adapted to a-silicon and polysilicon and are less suitable for Mo. Their viscosity is particularly suitable for the technique of drawing flat glass sheets and less so for that of drawing tubes for producing lamps. The same applies to the glasses which are disclosed by European patent publication 0,672,629 and U.S. Pat. No. 5,508,237.
In order to be used as glass for lamp bulbs which contain molybdenum components as electrode material or electrical lead material, the thermal expansion of the glass must be adapted to that of molybdenum, so that a tight, stress-free seal between the metal and the glass is achieved.
This means that the glass must have a higher expansion coefficient at its setting temperature (set point) than Mo, for example, the difference in expansion between Mo and the glass must be positive, in order to achieve a radial compressive stress in the glass which is beneficial to the sealing of Mo supply leads.
A further demand placed on a glass which is to be suitable for use as glass for lamp bulbs is that it be suitable for tube drawing. For this to be so, it must have sufficient crystallization stability.